Function global_output_source 

pub(super) fn global_output_source(
    _per_channel: &Arc<FileStore>,
    global: &Option<Arc<FileStore>>,
    mstore: &Arc<Store>,
    block_id: &str,
    filename: &str,
) -> Option<(Arc<FileStore>, String)>

If the agent’s GLOBAL transcript zone (agent:<defId>:current) has output, return (global_store, agent_zone) — preferred even when this channel’s local output is non-empty. Returns None when the block isn’t agent-anchored or is archived, there’s no global store, or the global zone is empty — callers then read the per-channel store keyed by block_id.

Only the agent output stream is globalized; every other file stays local.